Double-membrane triple-electrolyte redox flow battery design

ABSTRACT

A novel design has been invented for redox flow batteries. Different from the single-membrane, double-electrolyte redox flow battery as a basic structure, the design of the present invention involves double-membrane (one cation exchange membrane and one anion exchange membrane), triple-electrolyte (one electrolyte in contact with the negative electrode, one electrolyte in contact with the positive electrode, and one electrolyte positioned between and in contact with the two membranes) as the basic characteristic. The cation exchange membrane is used to separate the negative or positive electrolyte and the middle electrolyte, and the anion exchange membrane is used to separate the middle electrolyte and the positive or negative electrolyte. This particular design physically isolates, but ionically connects, the negative electrolyte and positive electrolyte. The physical isolation offers a great freedom in choosing redox pairs in the negative electrolyte and positive electrolyte, making high voltage of redox flow batteries possible. The ionic conduction not only makes the design functional, but also drastically reduces the overall ionic crossover between negative electrolyte and positive one, leading to high columbic efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No.61/660,182, filed Jun. 15, 2012 and incorporated herein by reference inits entirety for all purposes.

GOVERNMENT SUPPORT

This invention was made with government support under Grant Nos.DE-AR000009 and DE-AR0000346 awarded by ARPA-E project of the U.S.Department of Energy. The government has certain rights in theinvention.

FIELD OF THE INVENTION

The invention pertains to redox flow batteries that have adouble-membrane (for example, one cation exchange membrane and one anionexchange membrane) and a triple-electrolyte (for example, oneelectrolyte in contact with a negative electrode, one electrolyte incontact with a positive electrode, and one electrolyte positionedbetween and in contact with the two membranes) as the basiccharacteristic.

BACKGROUND

As an electrochemical cell, a redox flow battery (RFB) is a type ofrechargeable battery that stores electrical energy, typically in twosoluble redox pairs contained in external electrolyte tanks. Anion-selective membrane (either cation exchange membrane, CEM, or anionexchange membrane, AEM) is used to physically separate, but ionicallyconnect, the two electrolytes that dissolve the two redox pairs. Thescale of external electrolyte stored can be sized in accordance withapplication requirements. When needed, liquid electrolytes are pumpedfrom storage tanks to flow-through electrodes where chemical energy isconverted to electrical energy (discharge) or vice versa (charge).Different from other conventional battery systems, RFBs store electricalenergy in the flowing electrolytes. Therefore, the energy capacity andthe power rating are fundamentally decoupled: The energy capacity isdetermined by concentration and volume of electrolytes, while the powerrating is determined by the size and number of cells in stack. Thisunique feature, combined with its long cycle-life, low capital-cost,scalability, and independence from geographical/geological limitationsthat are faced by pumped hydro and compressed air technologies, makesRFB one of the most intrinsically attractive technologies in electricalenergy storage, especially in the field of renewable (e.g., wind orsolar) electricity generation where the intrinsic intermittency has tobe dealt with.

Since the first concept of RFB was put forward about 40 years ago (in1974), significant progress has been made and some RFB systems, e.g.,the all vanadium RFB (AV-RFB), have already been commercialized.However, RFBs have not reached broad market penetration yet because manychallenging problems remain unsolved. For example, the generally lowenergy and power density of RFB have been identified to be maindrawbacks when compared with other battery systems, which means moreelectrolyte/electrode materials are needed when certain energycapacity/power rating is required, negatively impacting theircost-effectiveness. Attempts have been made to increase the solubilityof active species by choosing alternative redox pairs or using differentelectrolytes, which can theoretically increase the energy density, butthese efforts do not improve the power density. On the other hand,efforts have been made to improve electrode performance by using betterelectrode designs or utilizing more active catalysts, which can increasethe power density, but not the energy density. The ideal and simplesolution would be the increase of RFB's cell voltage, which couldincrease the energy density and power density simultaneously.

A prior art RFB system 100 is shown in FIG. 1. Negative electrolyte 30flows through negative electrode (anode) 31 from negative electrolytesource 20 via pump 15. Positive electrolyte 40 flows through positiveelectrode (cathode) 41 from positive electrolyte source 25 via pump 16.Positive electrode 40 and negative electrode 30 are separated by asingle ion selective membrane 28. The RFB 100 may be connected to a gridinput/output processor 10.

The cell voltage is simply determined by the two redox pairs used, andoften the cation-based redox pairs (e.g., Co³⁺/Co²⁺ redox pair with+1.953 V standard electrode potential and Ce⁴⁺/Ce³⁺ one with +1.743 V,all the quoted potential values here and hereinafter calculated based onstandard thermodynamic conditions) have more positive electrodepotentials (ideally for the positive electrode of RFB) and theanion-based ones (e.g., Al(OH)₄ ⁻/Al with −2.337 V and Zn(OH)₄ ²⁻/Znwith −1.216 V) have more negative electrode potentials (ideally for thenegative electrode). The use of a single ion-selective membrane, eithera cation exchange membrane (CEM) or anion exchange membrane (AEM), incurrent RFB systems theoretically requires the same ionic type of redoxpairs in both positive and negative sides: either all cation-based redoxpairs (when AEM used) or all anion-based ones (when CEM used),fundamentally limiting their cell voltages. For example, the earliestRFB system, i.e., the iron-chromium RFB system (Fe/Cr-RFB,[(Fe³⁺/Fe²⁺)/(Cr³⁺/Cr²⁺)] with +1.18 V standard cell voltage) and thecurrently most popular RFB, i.e., the AV-RFB system ([(VO₂⁺/VO²⁺)/(V³⁺/V²⁺)] with +1.26 V) both belong to the all-cation-based RFBsystems. The polysulphide-bromine RFB system (S/Br-RFB, [(S₄ ²⁻/S₂²⁻)/(Br₃ ⁻/Br⁻)] with +1.36 V) is a typical all-anion-based RFB.Besides, the single ion-selective membrane also requires the same orsimilar (e.g., having the same cation but different anions when an AEMused, or having the same anion but different cations when a CEM used)supporting (or background) electrolyte in positive side and negativeone, which sometimes limits the choices of redox pairs and furthernarrows the available range of cell voltages. For example, although thezinc-cerium RFB system (Zn/Ce-RFB, [(Zn²⁺/Zn)/(Ce⁴⁺/Ce³⁺)]) can offer ashigh as 2.50 V standard cell voltage (the highest number reported amongall known aqueous RFB systems), it suffers a great hydrogen evolutionproblem in negative side (Zn²⁺/Zn). The reason is that the acidicsupporting electrolyte used (in both sides) creates a hugeover-potential (760 mV) for hydrogen evolution reaction (0 V standardelectrode potential of H⁺/H₂ at pH 0 vs. and −0.760 V standard electrodepotential of Zn²⁺/Zn).

In addition, the use of single ion-selective membrane makes the RFBsystems suffer from an irreversible counter-ion crossover that isanother challenging problem, because all ion-selective membranes are notperfect. They allow a very low, but measurable, rate of permeation ofcounter-ions through them (typically, 1% anion crossover for CEMs and1%-5% cation crossover for AEMs). When the counter-ions cross over themembrane, they will immediately react with the redox pairs in the otherside of electrolyte (so-called self-discharging) and never come back,resulting a loss in Coulombic efficiency, permanently reduction ofenergy capacity, and contamination of two electrolytes which willgreatly influence the performance of either side.

Thus, a suitable alternative to a single ion-selective membrane RFBsystem is needed.

BRIEF SUMMARY OF THE INVENTION

In an aspect, the invention provides a novel redox flow battery design,e.g., a double-membrane, triple-electrolyte (DMTE) based redox flowbattery design, comprising a first membrane; a second membrane; a firstelectrolyte positioned between and in contact with the first membraneand the second membrane; a second electrolyte in contact with the firstmembrane and a first electrode; and a third electrolyte in contact withthe second membrane and a second electrode; and wherein the firstelectrolyte and second electrolyte are different in term of at least onespecies of anion, and the first electrolyte and third electrolyte aredifferent in terms of at least one species of cation; and wherein thefirst electrode is a negative electrode (or a positive electrode) andthe second electrode is a positive electrode (or a negative electrode);and wherein the first membrane and/or the second membrane is selectedfrom the group consisting of a cation exchange membrane and an anionexchange membrane.

Another embodiment of the invention pertains to a redox flow batterycomprising: a cation exchange membrane having a first surface and asecond surface; an anion exchange membrane having a first surface and asecond surface; a first electrolyte positioned between and in contactwith the first surface of the cation exchange membrane and the firstsurface of anion exchange membrane; a second electrolyte in contact withthe second surface of the cation exchange membrane and a first electrode(e.g., a negative electrode); and a third electrolyte in contact withthe second surface of the anion exchange membrane and a second electrode(e.g., a positive electrode).

In another aspect, the second electrolyte comprises an anion-based redoxpair, such as an anion-redox pair selected from the group consisting ofan Al(OH)⁴⁻/Al redox pair, a Zn(OH)₄ ²⁻/Zn redox pair and a Co(CN)₆³⁻/Co(CN)₆ ⁴⁻ redox pair. In another aspect, the third electrolytecomprises a cation-based redox pair, such as a cation-based redox pairselected from the group consisting of a Co³⁺/Co²⁺ redox pair and aCe⁴⁺/Ce³⁺redox pair. In another aspect, one of the first, second andthird electrolytes comprises at least one of: cations based onhydronium, sodium, magnesium, potassium or calcium; or anions based onhydroxide, perchlorate, sulfate, phosphate, acetate, chloride, bromideor carbonate.

In yet another aspect, the invention provides a method of making a redoxflow battery comprising: partially surrounding a first electrolyte witha first membrane and a second membrane; b) partially surrounding asecond electrolyte with the first membrane and a first electrode; andpartially surrounding a third electrolyte with the second membrane and asecond electrode; wherein the first membrane and/or the second membraneis selected from the group consisting of a cation exchange membrane andan anion exchange membrane. In yet a further aspect, the inventionprovides a battery made by this method.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a conventional single-membrane double-electrolyte RFB.

FIG. 2 shows an illustration of a double-membrane triple-electrolyte(DMTE) RFB concept.

FIG. 3 shows a double-membrane triple-electrolyte RFB.

FIG. 4 shows a standard electrode potential for some redox pairs andcell voltage for different batteries.

FIG. 5 shows the open circuit voltage of an Al/Co-DMTE-RFB.

FIG. 6 shows the charge and discharge curves of an Al/Co-DMTE-RFB.

FIG. 7 shows the open circuit voltage of a Zn/Ce-DMTE-RFB.

FIG. 8 shows the charge and discharge curves of a Zn/Ce-DMTE-RFB.

FIG. 9 shows a continuous charge-discharge test of a Zn/Ce-DMTE-RFB for10 cycles.

FIG. 10 shows the efficiency calculation of a Zn/Ce-DMTE-RFB for eachcycle.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof. In the following description,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art, that the present invention may be practicedwithout some or all of these specific details. In other instances, wellknown process steps and/or structures have not been described in detailin order to not unnecessarily obscure the present invention.

The double membrane, triple electrolyte (DMTE) RFB systems describedherein can dramatically increase cell voltages and decrease ioniccrossover simultaneously by involvement of a double-membrane arrangement(one piece of CEM and one piece of AEM) that divides the RFB cell intothree compartments filled with triple-electrolyte (one in contact with anegative electrode, one in contact with a positive electrode, and thethird one in between the two membranes). By introducing one moreion-selective membrane, the two electrolytes in the negative side andthe positive side can be substantially separated but still remainionically conductive by the third electrolyte positioned between themembranes. This particular three-compartment design favorably bringsgreat freedom in selecting redox pairs as well as their supportingelectrolytes for both the negative side and the positive side, makinghigh cell voltage RFBs possible. Besides providing the function of ionicconduction, the middle electrolyte in between also serves as a great“buffer” that can significantly reduce the overall counter-ion crossoverbetween the negative side and the positive side, fundamentally solvingthe electrolyte contamination problem and providing great conveniencefor electrolyte separation and rebalance.

Specifically, the double-membrane triple-electrolyte design describedherein allows for a strongly basic negative electrolyte (high pH, e.g.at least 8, at least 9, at least 10 or higher) and a strongly acidicpositive electrolyte (low pH, e.g., not more than 6, not more than 5,not more than 4 or lower) to be used at the same time in the same redoxflow battery, where a neutral middle electrolyte is in between. As aresult, very negative redox pairs that are usually only stable in basicelectrolytes and very positive ones that are usually only stable inacidic electrolytes can be simultaneously incorporated into theDMTE-RFB, providing very high cell voltage and very low ionic crossoverat the same time. Not only can the cell voltage be increased, the sidereaction of hydrogen evolution in negative electrode can also besuppressed, as the standard electrode potential for hydrogen evolutionreaction is very negatively shifted in a basic electrolyte in comparisonwith an acidic electrolyte (e.g., from 0 V at pH=0 to −0.828 V atpH=14), thermodynamically extending the operational window of cellvoltage. Such a low counter-ionic crossover design overcomes the verychallenging electrolyte contamination problems that hamper thecommercial use of most of the current batteries.

FIG. 2 shows a DMTE-RFB system 200 wherein first electrolyte 60 may bepartially surrounded by a second electrolyte 50 and a third electrolyte70, wherein first electrolyte 60 may be separated from secondelectrolyte 50 by a first membrane 80, such as a cation exchangemembrane (CEM) 80, and wherein first electrolyte 60 may be separatedfrom third electrolyte 70 by a second membrane 90, such as an anionexchange membrane (AEM) 90. Second electrolyte 50 may be partiallysurrounded by a first electrode 35, such as a negative electrode (anode)35. Third electrolyte 70 may be partially surrounded by a secondelectrode 45, such as a positive electrode (cathode) 45.

FIG. 3 shows a DMTE-RFB 300 herein, and similar to FIG. 2, firstelectrolyte 60 may be partially surrounded by a second electrolyte 50and a third electrolyte 70, wherein first electrolyte 60 may beseparated from second electrolyte 50 by a first membrane 80, such as acation exchange membrane (CEM) 80, and wherein first electrolyte 60 maybe separated from third electrolyte 70 by a second membrane 90, such asan anion exchange membrane (AEM) 90. Second electrolyte 50 may bepartially surrounded by a first electrode 35, such as a negativeelectrode (anode) 35. Third electrolyte 70 may be partially surroundedby a second electrode 45, such as a positive electrode (cathode) 45.Second electrolyte 50 is flowed from second electrolyte source 51 viapump 17. Third electrolyte 70 is flowed from third electrolyte source 71via pump 19. First electrolyte 60 is flowed from first electrolytesource 61 via pump 18. DMTE-RFB 300 may be connected to a gridinput/output processor 11.

It will be understood to those skilled in the art that elements 35 and45 are referred as electrodes, but they may also include currentcollectors (not shown). The current collectors may be the same ordifferent material as the electrodes. It will be understood to thoseskilled in the art that electrodes/current collectors 35, 45 may havehigh specific surface area (e.g., be highly porous).

First, second and third electrolytes 60, 50, 70 are not particularlylimited and may comprise any suitable electrolyte or salt, such as thosebased on cations of hydronium, sodium, magnesium, potassium or calcium,or anions based on hydroxide, perchlorate, sulfate, phosphate, acetate,chloride, bromide or carbonate. First and second electrodes 35, 45 arenot particularly limited and may comprise any suitable electrodematerial, such as Al, Zn, Cu, Cd, Pb and C.

The DMTE-RFB systems described herein have great advantages overconventional single-membrane, double-electrolyte RFBs and offers highOCV, low ionic crossover, and suppressed hydrogen evolution. Thematerials used to construct the DMTE battery systems described hereinare not particularly limited and may be a myriad of materials, forexample, any materials selected from conventional or otherwise knownmaterials used for similar purposes in the energy arts. Such materialsinclude, but are not limited to, cation exchange membranes, anionexchange membranes, electrolyte solutes and solvents, compounds capableof providing the desired redox pairs, acids, bases, negative electrodes,positive electrodes, and the like. The DMTE-RFB systems described hereinhave a wide range of applications, especially for high voltage and lowionic crossover RFBs.

For example, an attractive candidate for a RFB system is analuminum-cobalt DMTE-RFB system (e.g., Al/Co-DMTE-RFB), configured as[(Al/Al(OH)⁴⁻)//(CO^(3+/CO) ²⁺)]. When compared to FIG. 2 or 3, the Alportion of the Al/Co-DMTE-RFB is comprised in second electrolyte 50 andthe Co portion of the Al/Co-DMTE-RFB is comprised in third electrolyte70. The Al/Co-DMTE-RFB system offers a very high cell voltage (4.29 Vstandard cell voltage), as it successfully combines the very negativeredox pair of Al/Al(OH)₄ ⁻ (−2.337 V standard electrode potential) inbase and the very positive redox pair of Co³+/Co² ⁺ (+1.953 V standardelectrode potential) in acid. Such a high standard cell voltage (4.29 V)is believed to be the highest one reported among all known RFB systems,which value is 1.7 times that of Zn/Ce-RFB systems (2.50 V), 3.2 timesthat of Polysulfide-bromide S/Br-RFB systems (1.36 V), 3.4 times that ofAll-Vanadium RFB systems (1.26 V), and 3.6 times that of Iron-ChromiumFe/Cr-RFB systems (1.18 V), as shown in FIG. 4. The very high standardcell voltage of the Al/Co-DMTE-RFB system is even higher than that oflithium ion batteries (around 3.5 V), suggesting a great potential ofthe design to rival other RFB technologies.

The test of open circuit voltage (OCV) for the Al/Co-DMTE-RFB system isshown in FIG. 5. As can be seen, a high initial OCV (3.54 V) and astable OCV (3.28 V) after 20 min is realized. Considering the verysluggish kinetics observed for the Co³+/Co²⁺ redox pair previously andthe completely non-optimized electrodes used, these OCV data arebelieved to be fairly consistent with the standard cell voltage (4.29V). Additionally, the Al/Co-DMTE-RFB system is functional, as shown bythe charge/discharge curves of FIG. 6. The expected discharge reactionsand charge reactions are shown in Eq. 1 and Eq. 2,respectively.

Co³⁺+e⁻→Co²⁺  Eq. 1 (a)

Al+4OH⁻→Al(OH)₄ ⁻+3e⁻  Eq. 1 (b)

Co²⁺→Co³⁺+e⁻  Eq. 2 (a)

Al(OH)₄ ⁻+3e⁻→Al+4OH⁻  Eq. 2 (b)

For the discharge process (the lower curve in FIG. 6), the cell voltageslightly and smoothly decreases from the initial 3.11 V to 2.84 V for 10min of discharge. An even longer time of discharge is also possiblealthough only 10 min of discharge operation is shown here as apreliminary experiment. The charge process has also been tried, and thecell voltage increases from the initial 3.25 V to 3.59 V for 10 min ofcharge (upper curve of FIG. 6). Clearly, the experimental demonstrationof the Al/Co-DMTE-RFB system confirms that such a design is feasible andsuccessful.

For example, another attractive candidate for a RFB system is azinc-cerium DMTE-RFB system (Zn/Ce-DMTE-RFB), configured as [(Zn/Zn(OH)₄²⁻)//(Ce⁴⁻/Ce³⁺)]. When compared to FIG. 2 or 3, the Zn portion of theZn/Ce-DMTE-RFB is comprised in second electrolyte 50 and the Ce portionof the Zn/Ce-DMTE-RFB is comprised in third electrolyte 70.

The Zn/Ce-DMTE-RFB system offers a standard cell voltage of 2.96 V, asit combines the negative electrode potential (−1.216 V) from theZn/Zn(OH)₄ ²⁻ redox pair and the positive one (+1.743 V) from theCe⁴⁺/Ce³⁺ redox pair. Such a high standard cell voltage is also higherthan those of all conventional aqueous RFB systems, e.g., higher thanthat of AV-RFB system (1.26 V) and that of Zn/Ce-RFB system (2.50 V, inspite of the strong concern of hydrogen evolution in negative electrodefor Zn/Ce-RFB system). The discharge and charge reactions arerepresented in Eq. 3 and Eq. 4, respectively.

Ce⁴⁺+e⁻→Ce³⁺  Eq. 3 (a)

Zn+4OH⁻→Zn(OH)₄ ²⁻+2e⁻  Eq. 3 (b)

Ce³⁺→Ce⁴⁺+e⁻  Eq. 4 (a)

Zn(OH)₄ ²⁻+2e^(−→Zn+)4OH⁻  Eq. 4 (b)

During charge process, the zincate anions are reduced to zinc metal andthe sodium cations are balanced from the middle compartment to thenegative compartment. In the meanwhile, cerium(III) cations are oxidizedinto cerium(IV) and the perchlorate anions are balanced from the middlecompartment to the positive compartment. During the discharge process,the opposite reactions and ion transfer directions will apply.

After being charged to reach a state of charge of 90%, the OCV ismonitored for 15 minutes. As seen in FIG. 7, it shows an initial OCV of3.14 V and quickly stabilizes to 3.10 V. These OCVs are higher than thestandard one (2.96 V), which is reasonable since the cell is in chargedstate (90% of state of charge). Clearly, such a high observed OCV againconfirms and verifies that the DMTE-RFB system is feasible andsuccessful.

Equally important, both a discharge operation and a charge operationhave been successfully achieved with a constant current (60 mA currentor 5 mA/cm² current density). As seen in FIG. 8, the cell voltage, inthe charge operation (upper curve of FIG. 8), slightly and very smoothlyincreases from initial 3.08 V to 3.17 V after 30 min of charge,indicating very high voltage efficiency (93%-96%). Different from theconventional Zn/Ce-RFB system where hydrogen evolution was found to be astrong concern (760 mV over-potential when acidic electrolyte used), thehydrogen evolution reaction is greatly suppressed in the Zn/Ce-DMTE-RFBsystem of the present invention as its over-potential drops from 760 mVin Zn/Ce-RFB to 388 mV in the inventive Zn/Ce-DMTE-RFB system (−1.216 Vof Zn/Zn(OH)₄ ²⁻ redox pair vs. −0.828 V of OH⁻/H₂ at pH=14). Indeed,the hydrogen evolution phenomenon has not been found during the wholedischarge operation as well as during the charge operation. In thedischarge operation (lower curve of FIG. 8), the cell voltage decreasesfrom the initial 2.88 V to 2.72 V after nearly 4 hours of discharge,showing a very steady voltage region, and then drops sharply afteravailable species being mostly consumed. The voltage efficiency fordischarge ranges from 92%-97%, almost equivalent to this number forcharge.

The discharge duration lasts for 3 hours and 56 minutes, very close tothe charge duration 4 hours, indicating high Coulombic efficiency. Theoverall Coulombic efficiency, voltage efficiency and energy efficiencyare calculated in Table 1. Combining the increased cell voltage,decreased ionic crossover, and suppressed hydrogen evolution, theZn/Ce-DMTE-RFB system of the present invention is clearly superior tothe conventional Zn/Ce-RFB system.

TABLE 1 Efficiency calculation for one charge-discharge cycle DischargeCharge Average discharge Average charge Columbic Voltage Energy time (s)time (s) Voltage (V) voltage (V) efficiency efficiency efficiency 1377714400 2.82 3.12 96% 90% 86%

The charge-discharge voltage curve at 5 mA/cm² is shown in FIG. 9,showing 10 successful continuous cycles without obvious Coulombicefficiency, voltage efficiency and energy efficiency change (as shown inFIG. 10 of efficiency for each cycle).

Example 1

An aluminum-cobalt DMTE-RFB system (Al/Co-DMTE-RFB), configured as[(Al/Al(OH)₄ ⁻)//(Co³⁺/Co²⁺)] was constructed. A three-compartment cellmade up of three plastic jars was designed and used as follows. Three 50ml plastic jars were put in series with a hole (a quarter inch ofdiameter) opened between adjacent two jars. The three jars, based onhalf-reaction inside, were assigned as negative, middle and positivecompartments. One piece of Nafion® 212 membrane (DuPont, 50 μmthickness) and one piece of Fumasep® FAA membrane (FuMa-Tech, 70 μmthickness) were used as the CEM and AEM, respectively. The CEM is putbetween the negative compartment and the middle compartment while theAEM is put between the middle compartment and the positive compartment,along with an O-ring to seal the conjunction part. Two clamps were usedto compress three jars tightly to avoid electrolyte leakage. Apotentiostat/galvanostat (Solartron 1287A) was used in both OCV anddischarge-charge cycle tests.

A solution that contained 3.76 M NaOH, 0.24 M NaAlO₂ and 0.05 M NaSnO₂was used as the negative electrolyte. A solution that contained 0.1 MCo(ClO₄)₂ and 2 M HClO₄ was used as the positive electrolyte, which wasprepared by dissolving CoCO₃ into perchloric acid. A 4 M NaClO₄ solutionwas used as the middle supporting electrolyte. A small piece of Al strip(ESPI Metals, 2 cm by 3 cm, 5N grade) and a small piece of graphite felt(SGL Group, 2 cm by 3 cm, Sigracell® GFA5 EA type) were used as thenegative electrode and the positive electrode, respectively. The cellwas first charged at 50 mA (or 8.3 mA/cm2 of current density) for 2.5hours and the OCV was tested for 20 min. The discharge-charge cycle isthen carried out for 20 min by setting current constant at 5 mA (or 0.83mA/cm2 of current density).

Example 2

A zinc-cerium DMTE-RFB (Zn/Ce-DMTE-RFB), configured as [(Zn/Zn(OH)₄²⁻)//(Ce⁴⁺/Ce³⁺)] was constructed. A three-compartment cell made up ofthree acrylic flow channels was designed and used as follows. Three 5 cmby 6 cm rectangular channels were put in series with membranes inbetween. The three channels, based on half-reaction inside, wereassigned as negative, middle and positive compartments. One piece ofNafion® 1135 membrane (DuPont, 87.5 μm thickness) and one piece ofFumasep® FAA membrane (FuMa-Tech, 70 μm thickness) were used as the CEMand AEM, respectively. The CEM is put between the negative compartmentand the middle compartment while the AEM is put between the middlecompartment and the positive compartment, along with silicone gasket toseal the conjunction part. The positive electrode and negative electrodeare each put next to its corresponding compartment, respectively. Twoclamps were used to compress the three channels and electrodes tightlyto avoid electrolyte leakage. Electrolytes are stored outside thechannel in three tanks and delivered by peristaltic pump (Masterflex®L/S® 100RPM). The working flow battery set-up is apotentiostat/galvanostat (Solartron 1287A) and was used in both OCV anddischarge-charge cycle tests.

The negative electrolyte contained 3 M NaOH and 0.5 M Na_(2[Zn(OH) ₄)].A solution that contained 0.5 M Ce(ClO₄)₃, 2 M HClO₄ was used as thepositive electrolyte, which was prepared by dissolving Ce₂(CO₃)₃ intoperchloric acid. The middle electrolyte used was 4 M NaCIO₄ solution.The volume for each electrolyte used in test is 30 ml. A rectangularcopper plate (ESPI Metals, 5 cm by 6 cm, 3N grade) was used as negativecurrent collector. Before the experiment, the copper was rinsed withacetone and deposited with a layer of cadmium according to the method inreference. Graphite based bipolar plate (SGL group, 5 cm by 6 cm,Sigracet® TF6 type) was used as positive current collector. Graphitefelt (SGL Group, 3 cm by 4 cm, Sigracell®GFA5 EA type) was used aspositive electrode and compressed by plastic frame to contact bipolarplate. The cut-off voltage for charge and discharge are 3.24 and 1.8respectively. The discharge-charge cycle was carried out at constantcurrent density at 60 mA (or 5 mA/cm² of current density) with flow ratefor all three electrolytes at 20 ml/min.

The DMTE-RFB systems described above may have other configurationsbesides those of acid/neutral/base configurations and are not limitedthereto. Hydroxide ions play the role of ligand to tune the electrodepotential of, for example, a Zn(II)/Zn or Al(III)/Al system, morenegative. Various ligands and central ions may be used for a similarrole and may be combined and implemented in a DMTE-RFB system. Table 2lists some possible candidates and combinations of negative, middle andpositive electrolyte.

TABLE 2 Negative, middle and positive electrolyte choices (electrodepotential from reference) Negative electrolyte Middle electrolytePositive electrolyte [Co(CN)₆]³⁻/[Co(CN)₆]⁴⁻ (φ_(N) = −0.83 V) Na₂SO₄Co³⁺/Co²⁺ (φ_(P) = 1.95 V) [Co(EDTA)]⁻/[Co(EDTA)]²⁻ (φ_(N) = 0.37 V)NaClO₄ Ce⁴⁺/Ce³⁺ (φ_(P) = 1.74 V) [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ (φ_(N) = 0.36V) VO₂ ⁺/VO²⁺ (φ_(P) = 1.00 V) [Cr(CN)₆]³⁻/[Cr(CN)₆]⁴⁻ (φ_(N) = −1.28 V)Fe³⁺/Fe²⁺ (φ_(P) = 0.77 V) Zn(OH)₄ ²⁻/Zn (φ_(N) = −1.22 V) Al(OH)₄ ⁻/Al(φ_(N) = −2.34 V)

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and substituteequivalents, which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and apparatuses of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, permutations, and substitute equivalents as fallwithin the true spirit and scope of the present invention.

What is claimed is:
 1. A redox flow battery comprising: a) a cationexchange membrane having a first surface and a second surface; b) ananion exchange membrane having a first surface and a second surface; c)a first electrolyte positioned between and in contact with the firstsurface of the cation exchange membrane and the first surface of theanion exchange membrane; d) a second electrolyte in contact with thesecond surface of the cation exchange membrane and a first electrode;and e) a third electrolyte in contact with the second surface of theanion exchange membrane and a second electrode.
 2. The redox flowbattery of claim 1, wherein the first electrolyte and second electrolyteare different in term of at least one species of anion, and the firstelectrolyte and third electrolyte are different in terms of at least onespecies of cation.
 3. The redox flow battery of claim 1, wherein thefirst electrode is a negative electrode and the second electrode is apositive electrode; or wherein the first electrode is a positiveelectrode and the second electrode is a negative electrode.
 4. The redoxflow battery of claim 1, wherein the second electrolyte comprises ananion-based redox pair or a cation-based redox pair.
 5. The redox flowbattery of claim 1, wherein the second electrolyte comprises a redoxpair selected from the group consisting of an Al(OH)₄ ⁻/Al redox pair, aZn(OH)₄ ²⁻/Zn redox pair and a Co(CN)₆ ³⁻/Co(CN)₆ ⁴⁻ redox pair.
 6. Theredox flow battery of claim 1, wherein the third electrolyte comprisesan anion-based redox pair or a cation-based redox pair.
 7. The redoxflow battery of claim 1, wherein the third electrolyte comprises acation-based redox pair selected from the group consisting of aCo³⁺/Co²⁺ redox pair and a Ce⁴⁺/Ce³⁺ redox pair.
 8. The redox flowbattery of claim 1, wherein the second electrolyte comprises an Al(OH)₄⁻/Al redox pair and the third electrolyte comprises a Co³⁺/Co²⁺ redoxpair.
 9. The redox flow battery of claim 1, wherein the secondelectrolyte comprises a Zn(OH)₄ ²⁻/Zn redox pair and the thirdelectrolyte comprises a Ce⁴⁺/Ce³⁺ redox pair.
 10. The redox flow batteryof claim 1, wherein the second electrolyte comprises a Co(CN)₆³⁻/Co(CN)₆ ⁴⁻ redox pair and the third electrolyte comprises a Co³⁺/Co²⁺redox pair.